The feature, called "Kill Zeus," apparently removes the Zeus software from the victim's PC, giving Spy Eye exclusive access to usernames and passwords.

Zeus and Spy Eye are both Trojan-making toolkits, designed to give criminals an easy way to set up their own "botnet" networks of password-stealing programs. These programs emerged as a major problem in 2009, with the U.S. Federal Bureau of Investigation estimating last October that they have caused $100 million in losses.

Trojans such as Zeus and Spy Eye steal online banking credentials. This information is then used to empty bank accounts by transferring funds to so-called money mules -- U.S. residents with bank accounts -- who then move the cash out of the country.

Sensing an opportunity, a number of similar Trojans have emerged recently, including Filon, Clod, and Bugat, which was discovered just last month.

Spy Eye popped up in Russian cyber crime forums in December, according to Symantec Senior Research Manager Ben Greenbaum.

With its "Kill Zeus" option, Spy Eye is the most aggressive crimeware, however. The software can also steal data as it is transferred back to a Zeus command-and-control server, said Kevin Stevens, a researcher with SecureWorks. "This author knows that Zeus has a pretty good market, and he's looking to cut in," he said.

Turf wars are nothing new to cyber criminals. Two years ago a malicious program called Storm Worm began attacking servers controlled by a rival known as Srizbi. And a few years before that, the authors of the Netsky worm programmed their software to remove rival programs Bagle and MyDoom.

Spy Eye sells for about $500 on the black market, about one-fifth the price of premium versions of Zeus. To date, it has not been spotted on many PCs, however.

Still, the Trojan is being developed quickly and has a growing list of features, Greenbaum said. It can, for example, steal cached password information that is automatically filled in by the browser, and back itself up via email. "This is interesting in its potential, but it's not currently a widespread threat at all," he said.